changed use of promote_args in agrad/partials_vari to return_type from meta/traits
changed use of promote_args in agrad/partials_vari to return_type from meta/traits

stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math